Etymology

atto-

Today it means: In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 10⁻¹⁸ (short scale quintillionth or long scale trillionth). Full definition →

Danish atten
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The story

  1. Danish · derived fromatten“eighteen”
  2. English todayatto-

From Danish atten (“eighteen”), with reference to the (negative) exponent in the relevant power of ten; the same principle also explains femto-.

Etymology from Wiktionary, CC BY-SA 4.0.